In general, it is often desirous to utilize filters in display technology and other lighting systems to attenuate or accentuate particular ranges of the electromagnetic radiation. For example, certain displays and visual equipment may accentuate particular colors in the visible light spectrum and attenuate other colors in the non-visible and the visible light spectrum. Accordingly, these displays can utilize filters to provide accentuation and attenuation for certain wavelengths of light.
In one particular filter application, displays and other equipment utilized in transportation, sports and military activities are often employed in tandem with night vision equipment. These displays and equipment conventionally utilize a filter to accommodate the night vision equipment. Issues related to the use of displays and night vision equipment are described below with reference to an aviation application, although the below-mentioned issues are relevant to any applications of displays, night vision equipment, optical systems, diagnostic equipment, or lighting systems requiring attenuation or accentuation of certain wavelengths of electromagnetic radiation.
Certain aviation displays are color displays that are utilized with night vision imaging systems (NVIS). These displays provide visual information to captains, pilots, drivers and operators of ships, aircraft, and vehicles. Hand-held displays also require NVIS compatibility for various military and law enforcement applications. The viewer of the color display often wears NVIS goggles at the same time he or she observes information from the color display.
Conventional NVIS goggles are sensitive to light in the infrared, near infrared, and visible red spectrum (wavelengths of light). NVIS goggles are typically sensitive to light between 425 nm and 1000 nm wavelengths. At 600 nm, the sensitivity rapidly increases and reaches a peak at 760 nm. The near infrared sensitivity of NVIS goggles allow the pilot or person wearing the goggles to see objects which cannot ordinarily be seen by the naked eye, but this same sensitivity can create night vision goggles (NVG) compatibility problems with cockpit displays. The compatibility issues fall into three categories. Category 1, 2 & 3 are, respectively, display emissions that are directly in the NVG's field of view, display emissions reflected into the NVG's field of view or display emissions diffusely scattered into the NVG's field of view. Category 1, 2 or 3 display emissions cause loss of contrast in the scene being viewed with the NVG. The contrast reduction leads to limited viewability and impaired object recognition, and it is known as NVG blooming or NVG flare.
The bloom effect is undesirable for two reasons. First, the bloom effect prevents the NVG wearer from seeing the operational environment clearly and in fine detail. Second, the night vision goggles require a certain amount of time to be reset after a bloom effect event. Accordingly, the bloom effect is undesirable when operating a vehicle or aircraft in night vision conditions.
Conventional avionic displays designed to be utilized with NVIS equipment generally are restricted to a narrow emission, such as, single color (e.g., green) displays. The narrow emission is chosen so that it does not interfere with NVIS equipment. However, the restriction to the narrow emission significantly reduces the information content and limits the symbology provided on the displays. Further, it is difficult to highlight and differentiate large amounts of information on the display if the display is restricted to a single color.
Other conventional avionic systems have included color displays that include an NVIS filter. The color display operates in two modes: an NVIS mode (e.g., low luminance) and a daylight mode. The NVIS filter is provided between a light source used in the NVIS mode and an optical shutter, such as a liquid crystal display. The filter prevents emissions that cause NVIS equipment to bloom.
One type of conventional NVIS filter is generally comprised of glass or other material supplemented by thin dielectric film coatings that attenuate infrared emissions or transmissions. This type of conventional NVIS filter typically uses a thin film, multi-layer dielectric to obtain a sharp cutoff, with the knee starting between 600 nm to 630 nm. Additionally, they use an absorptive substrate to attenuate longer wavelength emissions.
Certain conventional active matrix liquid crystal displays (AMLCD) utilize two basic approaches for NVIS compliant backlighting. Both approaches have disadvantages associated with cost, space, and display quality.
In the first approach, a single lighting source is utilized in combination with a large area infrared (IR) cutoff filter (a single mode AMLCD). The ER cutoff or NVIS filter is typically a thin film dielectric stack having a surface area equal to the surface area of the AMLCD. The NVIS filter, which is disposed in the optical path, can cost a significant amount. The conventional NVIS filter can cause undesirable display performance, such as reduced backlight efficiency, red de-saturation and reduced display luminance. The conventional thin film dielectric stack also can cause viewing angle performance issues.
In NVIS required LCD applications containing LED backlights utilizing the use of NVG, there is a need as discussed above to provide filtering of those LED devices that are in use at night. Typically, this has been accomplished using one of two methods. Using a first method, all LEDs (i.e., day-mode LEDs and night-mode LEDs) are filtered using an NVIS filter. This approach requires sacrifices in day-mode performance (luminance reduction) or increased power in order to achieve the required night-mode filtering. Using a second method, each night-mode LED is filtered with its own lens cap filter or filter coating. This is both expensive and difficult to assemble.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.